Heat up hot water by Gas or Electricity?

Tgrewal

Due to recently getting an electric car, I switched my energy supply to Octopus. I am on the Intelligent Octopus (IO) tariff for my electric and Octopus Flexible tariff for my gas.

Every day in the early hours of each morning, I heat up my hot water (Megaflo hot water tank) for 2 hours via the 30kW gas boiler. The off-peak electricity rate (23:30 - 05:30) is 10p kWh and the gas rate is 10.41p kWh. 

Stupid question but if I were to heat up my hot water for 2 hours via the electrical immersion heater during the off-peak hours, would it cost me the same in electricity as it would in heating up the water via gas for 2 hours, or would it roughly be the same since both, the gas and electricity per kWh rate is roughly the same?

matt_drummer

Electric will be cheaper as it is 100% efficient and your gas boiler is not, and in any case the electricity cost per Kwh is less than the gas.

Whether you need to heat for the same amount of time using electricity will depend on your immersion heater power.

It may take longer with electric but the cost will be less assuming the water in the tank gets to the same temperature as it did with gas.

markin

The boiler will at be around 90-98% efficiency so electricity will be cheaper, also not burning gas would be greener even if costs were dead on, if you care about that.

Mstty

markin said:

The boiler will at be around 90-98% efficiency so electricity will be cheaper, also not burning gas would be greener even if costs were dead on, if you care about that.

Is this factually correct as I keep reading the actual average is 85% for gas boilers (perhaps this is all ages combined) but would love to know the real figure as it does have a bearing when calculating cost.

matt_drummer

Mstty said:

markin said:

The boiler will at be around 90-98% efficiency so electricity will be cheaper, also not burning gas would be greener even if costs were dead on, if you care about that.

Is this factually correct as I keep reading the actual average is 85% for gas boilers (perhaps this is all ages combined) but would love to know the real figure as it does have a bearing when calculating cost.

It's difficult to know but I would say that 98% efficiency is extremely optimistic.

I used 90% when I estimated my heat loss.

Tgrewal

thanks @matt_drummer. I have noticed in the past that heating the hot water via the immersion heater takes longer than heating via gas. I would think that I would probably need to allow 3-4 hours heating via electric compared to 2 hours via gas. Based on this, would electric cost more?

matt_drummer

No, not if the water is heated to the same temperature.

If your immersion heater is 3Kw, in four hours it will use 12Kwh and if that is what it took to heat your water then it took 12 Kwh of energy to heat up.

@ 10p per Kwh that is £1.20


Your gas boiler is more powerful than the immersion heater, my gas boiler is 28 Kw

Your gas boiler will heat the tank much more quickly, even though it is programmed to be on for two hours it will switch off when the tank thermostat tells it to.

So your gas boiler needs to provide 12 Kwh of heat, to do this at 90% efficiency it will use 13.33 Kwh of gas (12/0.90) which will cost you £1.39 (13.33 Kwh @ 10.41p per Kwh).

So, electricity is cheaper but it is only a small difference.

If it was me, I would leave it to my gas boiler.

Mstty

matt_drummer said:

Mstty said:

markin said:

The boiler will at be around 90-98% efficiency so electricity will be cheaper, also not burning gas would be greener even if costs were dead on, if you care about that.

Is this factually correct as I keep reading the actual average is 85% for gas boilers (perhaps this is all ages combined) but would love to know the real figure as it does have a bearing when calculating cost.

It's difficult to know but I would say that 98% efficiency is extremely optimistic.

I used 90% when I estimated my heat loss.

I am about the same 90% for condensing as let's face it most are not setup correctly when installed and very few (this boards regulars excluded) 

Non condensing 70-80%?

So overall I might stay with 85% as. General rule of thumb

DullGreyGuy

Tgrewal said:

thanks @matt_drummer. I have noticed in the past that heating the hot water via the immersion heater takes longer than heating via gas. I would think that I would probably need to allow 3-4 hours heating via electric compared to 2 hours via gas. Based on this, would electric cost more?

Thats because your electric heater is less powerful but thats irrelevant. You need to consider draw x time x efficiency. 4 hours at 3kw is the same as 2 hours at 6kw or 1hr at 12kw. 

The issue is that gas is less efficient than electricity and so even if both were 10p/kwh then to actually put 1kw of energy into the cold water would cost 10p for electricity and circa 11.1p for gas (assuming a 90% efficiency). For those on other tarriffs the fact that gas is much cheaper per kwh than electricity means the lower efficiency still makes gas the cheaper option.

Alnat1

You could always try heating with gas for only 90 minutes and see if you have enough hot water. That's one way to save a bit.

k_man

@Tgrewal

A few points to consider:

Does your immersion heater have a timer control?
Systems with gas boilers often don't have a timer for the immersion heater, which is more for emergency/occasional use.

The immersion will use it's own thermostat for the temperature to heat to, so you may find the water is hotter than with gas, so careful if you are used to washing hands under just the hot tap.

Systems with both gas and immersion heaters usually have the immersion at the top, as above, more for a quick boost of hot water.
This can mean that the whole tank is not fully heated, especially if there is no hot water usage to circulate/mix the water.
As a result, while the hot water may be a higher temperature, there may be less of it.